REGIONAL CHANGES IN CEREBRAL HEMODYNAMICS AS A RESULT OF A VISUAL STIMULUS MEASURED BY NEAR-INFRARED SPECTROSCOPY

Near infrared spectroscopy (NIRS) is used to measure global changes in cerebral haemodynamics. We have adapted the technique to measure regi onal changes in response to a visual stimulus. Ten volunteers were exp osed to a computer generated visual stimulus designed to activate a la rge area of the visual cortex, including V1, V2, V3, V4 and V5. The st imulus was on for 30 s and off for 30 s. Changes in the concentrations of oxyhaemoglobin ([HbO(2)]) and deoxyhaemoglobin ([Hb]) were measure d using a commercial spectrometer (NIRO500), over the occipital cortex . The data were summed over ten cycles. As a control, the experiment w as repeated over the frontal cortex. For each subject [HbO(2)] increas ed during stimulation, and decreased when the stimulus was off. The me an (+/-s.e.m.) change in [HbO(2)] was 0.54 +/- 0.14 mu mol l(-1). The change in total haemoglobin concentration, given by [HbO(2)] + [Hb] wa s 0.61 +/- 0.21 mu mol l(-1), equivalent to a rise in cerebral blood v olume of 0.04 +/- 0.01 ml 100 g(-1) which is about 2% of the total cer ebral blood volume. There was no significant change in [HbO(2)] over t he frontal cortex, implying that the changes in blood volume originate d in the occipital lobe. This demonstrates that NIRS provides a non-in vasive method of measuring regional changes in cerebral haemodynamics as a result of visual stimulation.